Prime mover dynamo plant having a speed droop characteristic



Mardl 1965 F. E. VANDAVEER, JR 3,176,142

PRIME MOVER DYNAMO PLANT HAVING A SPEED DROOP CHARACTERISTIC Filed 001;.11, 1960 SPEED NORMAL SPEED IN V EN TOR.

%NORMAL FREDERICK E. VANDAVEER, JR.

0 5 0% 160% 50 LOAD BY F/G.

United States Patent 3,176,142 PRIME MOVER DYNAMO PLANT HAVING A SPEEDDROOP CHARACTERISTIC Frederick E. Vandaveer, In, North Chili, N.Y.,assignor to Taylor Instrument Companies, Rochester, N.Y., a

corporation of New York Filed Oct. 11, 1960, Ser. No. 62,017 7 Claims.(Cl. 290-40) This invention relates to the art of automaticallycontrolling a system or process of the type wherein it is desirable thatsystem or process response to control efforts have a droopingcharacteristic.

For example, in an electrical power generating system, wherein aplurality of motor-generator units supply in parallel a common, varyingload, it is necessary to assure that each unit maintains its propershare of the load.

In a typical unit, such as a gas turbine driving an AC. generator, thefrequency and power output is a function of turbine speed, and the basiccontrol effect of the usual control system is to automatically resistchanges in turbine speed. While it is possible to exercise automaticspeed control such as to make the unit to operate very closely at thesame speed for different loads, i.e., isochronously, this may createload sharing problems. For example, if one unit of a system tends torespond to load changes faster than its fellows, it will tend to takemore than its share of the load, upon load increase, or less than itsshare of the load, upon load decrease. In the past, this problem of loadsharing has been dealt with by taking advantage of the phenomenon knownas droop.

In a typical system, turbine speed is controlled by a fuel valvepositioned by a servo motor. The servo motor is, in turn, controlled bya hydraulic pilot having an element the position of which depends on theposition of a governor fly-weight element relative to the position ofsome linkage which may be set to various positions depending on theturbine speed desired at a given load. The said linkage is also arrangedso that, upon speed changes, the speed control effect is fed back to thelinkage in such a way as to make the effective speed setting of thelinkage closer to the actual speed of the turbine then obtaining. Thus,if turbine speed drops due to load increase, the speed set point iseffectively dropped so that turbine stabilizes out at a speed less thanthe original setting of the said linkage.

Hydraulic controllers of the type described above have reached a highdegree of perfection in function and structure. In particular, beinghydraulic, they are inherently fast-acting devices, and, moreover, areself-contained packages including centrifugal governor and hydraulicpower supply. However, they are also precise, complex devices whichrequire a high degree of skill and elaborate facilities inmanufacturing, repair and maintenance. Hence, the user, upon malfunctionof the package, seldom has any recourse but to return the unit to themanufacturer or turn it over to a specialist for repair or servicing.

Furthermore, in present day generator systems, the

individual speed controls of the motor-generator units are small partsof a much larger overall control system, and the speed controls arenecessarily integrated with the other parts of the overall controlsystem.

However, hydraulic speed controls are not well-suited for such service,because such service would require considerable length of connectinghydraulic piping and interconnection of hydraulic circuitry. Moreover,except for small units which can go from full load to no load in amatter of seconds, the speed of hydraulic action is not important. Thepossibilities of leakage, and the design problems inherent in designinghydraulic circuitry involving hundreds of feet of liquid-carrying pipeto be used in anenvironment having a varying heterogeneous tem-3,176,142 Patented Mar. 30, 1965 perature distribution and incorporatingthe compact, carefully designed hydraulic circuitry of individual speedcontrol units, more or less rule out a larger, overall hydraulic system.

On the other hand, in extensive control systems, the trend is to usepneumatic apparatus, and, hence, it would be convenient to the systemdesigner if hydraulic speed control units were replaced by pneumaticversions thereof.

According to my invention, I provide a pneumatic speed control apparatuswith droop, that does all that its hydraulic counterpart can do. Inaddition, my novel control apparatus is cheaper in initial cost, modularin nature, can be serviced and repaired by the user, and in general, hasall the advantages of pneumatic apparatus in respect of the consequencesof leakage, environmental temperature, and so on, as compared tohydraulic apparatus.

By modular in nature, I refer to thefacts that pneumatic systems aregenerally a collection of relays strung together with appropriatepiping, and that a few basic relay types can provide a much largernumber of functions, are interchangeable, and can be stocked with theview in mind of being able to use them in different parts of the overallcontrol system. This is to be contrasted with the selfcontainedhydraulic speed control package, which is an individual in itself and ingeneral, must be so treated when it comes to service, repair andadjustment.

Turning now to the drawings:

FIGURE 1 is a graph of actual speed versus actual load in percentrelative to a given load to be driven at a predetermined speed;

FIGURE 2 is schematic diagram of a plural unit electrical powergenerating system; and

FIGURE 3 illustrates my novel control system as ap plied to one of theunits of the system shown in FIGURE 2.

Droop, or offset, as it often is termed in control theory, is aninherent characteristic of simple proportional control of a processhaving a certain amount of inertia, capacity, lag, or othercharacteristic, the end result of which is to cause the process tostabilize out at a point that is a function of the load represented byprocess behavior. For example, in a simple governor controlled turbinesystem wherein a governor tends to open or close the fuel or steam valveof the turbine in proportion to its speed, the speed of the turbine willbe a function of the load on the turbine, a characteristic oftenexpressed as the regulation of the turbine, i.e., the percentage changein speed for a range of percentages of a given load relative to a givenspeed at said given load.

This characteristic is illustrated in FIGURE 1, from which it will beseen from the said figure, that at more than of load, the turbinestabilizes out a speed less than that for 100% of load. Since the powergenerated into a load is a function of speed, the regulation of theturbine opposesthe tendency to pick-up and drop load upon load changes,as compared to isochronous action (which would be represented by thehorizontal Normal Speed line in FIGURE 1).

FIGURE 1 also illustrates how the turbine goes from one stabliizationpoint to another by means of an Actual Speed line (which may be curved,but is here shown as straight, for convenience) representing the lows ofthe stabilization points of the system for different loads. For example,to get from point A, Normal Load, to point B, 50% of Normal Load, theturbine speed varies along the dashed line running from A to B duringthe time it takes the turbine to stabilize out at the speedcorresponding to point A.

Again, as the turbine goes from 50% of Normal Load to of Normal Load,turbine speed varies as indicated by the dashed line from point B topoint C.

The speed fluctuation indicated by the dashed line curves is generallycontrolled so as to keep the area between them and the Actual Speed lineat a minimum, hence, one finds that the typical speed controller isdesirably a so-called multi-response device, i.e., one that not onlymeasures out its control elfectin proportion to the deviation of actualspeed from normal speed, but also in accordance with the rate of changeof deviation and the time integral of deviation. This lastcharacteristic of the controller, known as reset action, has thecharacteristic of tending to prevent droop or olfset and, therefore,where reset action is involved, the necessary droop is obtained byvarying the set point of the speed control system.

Turning again to FIGURE 1, the Actual Speed line represents the resultof droop in a speed control system under varying load. Supposing someNormal Load to be chosen at which the generator unit will require to berun at a speed given by the Normal Speed line in FIGURE 1, if the loadvaries in a range from 50% of normal to 150% of normal, the regulationof the system is represented by the difference in the 50%of-Normal-Loadspeed and the 150%-of-Normal-Load speed, i.e., the speed differencegiven by the vertical distance between points B and C on the ActualSpeed line. As is known in the art, it such regulation or droop isadjusted to suit the load-following characteristic of eachmotor-generator unit in a system of units in parallel driving a commonvariable electrical load, each unit will carry its share of the varyingload at all times. In typical cases, the amount of droop needed rangesup to about 10% of Normal Speed.

As indicated by what was described initially of FIG- URE l, thespeed-load picture in this figure is somewhat idealized. Nevertheless,given the particular speed-load characteristic of a particular unit, itis possible to establish the amount of droop in the speed control systemof that unit such as will enable the unit to follow load changes in theproper proportion along with its fellows, each of which has beenprovided with the appropriate droop calculated to make it cooperatelikewise in the generating system.

FIGURE 2 shows schematically the sort of generating system envisagedherein. Such systems comprise a plurality of generators, of number n,say, i.e., G G as indicated in the figure.

Generator G is driven by a motor T, say a gas turbine, having a fuelintake 1 controlled by a flow control valve V, the exhaust of theturbine being shown at 2. Turbine T has a shaft 3 that rotates thearmature (not shown) of generator G and the generator has outputterminals 4 and 5 at which electric power is fed into lines L, connectedto one or sundry electrical power consuming means (not shown) of suchnature that the electrical demand on the generating system varies fromtime to time,

To control the output of generator 6,, a control system is providedincluding speed sensing device S, connected to shaft 3 and emitting asignal in proportion to turbine speed, which signal is transmitted via aconnection 6 to a controller C.

Controller C receives the speed signal and emits a control signal whichis relayed via override controls 0 and connections 7, 8, and 9 to valveV.

Override controls 0, for purposes of this application, are relay devicesthat normally repeat the control signal emitted by controller C into theconnections 8 and 9, and hence, do not exist except in specialcircumstances. In this case, these special circumstances may includetemperature, pressure, and other conditions such as would signifydangerous or otherwise undesirable operating conditions. Merely by wayof example, override controls 0 are illustrated as connected to inlet 1and exhaust 2 of turbine T by connections 10 and 11 for the purpose ofsensing temperatures and responding to critical values of temperature toshut down the fuel flow. Assuming a biased-closed, signal-to-open valveV, override controls 0 will be constructed to cut-off the signalconnection between valve V and controller C upon occurrence of animproper temperature. Otherwise, override controls 0 merely relay thecontrol signal on to valve V. Since such overriding of the controller Cis known in the art, it is of no concern to this application to indicatethe exact manner of operation and construction of override controls O.

Generator G would likewise be fitted out as is generator G and isconnected to lines L, and, therefore, neither it nor its control systemneed be described any further, and so on for each of whatever number ofgenerators may be connected to lines L. It is to be noted, however, thatthe generators involved (and their motors, etc.) need not be identicalas to structure, capacity, etc. Each motor generator unit must becapable of being drooped, however, in order to assure proper loadsharing in the face of varying load.

FIGURE 3 shows one embodiment of my novel pneumatic speed control systemwith droop. In the figure, the main components of the system are shownto be pneumatic relay devices 20 and 40.

Relay device 20 is a conventional controller having proportional, rateand reset action, and comprises bellows 21, 22, 23 and 24, the near endsof which are connected together by means of a cross bar 25, the lengthsof the bellows being parallel and their far ends being fixed to a commonsupport (not shown). The said bellows are substantially identical andare equispaced about a circle divided into four quadrants by the arms ofcross bars 25. Bellows 21 to 24 are so arranged (as by providingsuitable adjustable biasing springs, not shown, between crossbar andsaid common support), that with equal pressure in opposite bellows thecrossbar 25 lies in a predetermined reference plane with each bellowssubstantially equally extended. A flexible, inextensible element 25A,such as a wire connected between said common support and the center ofcrossbar 25, allows crossbar 25 to be deflected by said bellows withoutbeing displaced bodily away from the said common support.

A pipe 60, corresponding to connection 6, FIGURE 2, connects bellows 21to speed transmitter S, adapted to measure the rate of rotation of shaft3, FIGURE 1, and to establish an air pressure in pipe 6%} and bellows21, the value of which corresponds to the measured rate of rotation.Such devices are well known in the art, hence, transmitter S needs nofurther description.

Bellows 23 and 24 are connected together by piping 26 and adjustablerestrictors 27 and 28, and pipe 70 connects piping 26, betweenrestrictors 27 and 28, to an override control 0. A further pipe connectsthe last said override control 0 to a further override control 0, andthis latter is connected to valve V by means of a pipe 96. Piping '70,8t and 90, obviously corresponds to connections 7, 8 and 9 of FIGURE 2.Likewise, valve V is in the fuel intake 1 of the turbine T.

To provide a control air pressure for pipe 70, there is provided abooster relay 30 connected at its output by pipe 29 to pipe 70. As shownby the symbol A.S., air under pressure is supplied via pipe 31 to relay3%) and, via a restrictor 32, to piping 33 and to a pipe 34 which isconnected between piping 33 and relay 3%). It is not necessary todescribe booster relay 3G in detail, since in accordance with prior artpractice, it is merely a pressure amplifier that responds to the levelof pressure in pipe 34 to establish a pressure in piping 29, 70, 26 andbellows 23 and 24 corresponding to the pressure in pipe 34. The outputpressure of the booster will be some definite multiple or fraction ofthe pressure in pipe 34, generally 121, since the booster relay is usedmainly for its characteristic of being able to establish its outputpressures in terms of air volume output much greater than thecorresponding air volume change at its input, i.e., in pipe 34. Suchrelays are well known in the art and need not be further described orillustrated.

To establish a control pressure in line 7% of the desired sort, thereare provided a nozzle 35 connected to pipe 33,,

and a circular cross section baffle rod 36. Baffle rod 36 is mounted atthe center of crossbar 25, with its length normal to the plane ofcrossbar 25.

Nozzle 35 is supported at the side of rod 36 with its opening (notshown) facing the side of the rod. Hence, within a certain spacingrange, rod 36 will obstruct flow through nozzle 35 in accordance withthe separation between nozzle opening. and the next adjacent rodsurface. Therefore, change in bafile-nozzle spacing will vary thepressure in pipes 33, 34 and 26.

A plate 37 supports nozzle 35 in the attitude described, and is itselfsupported by a rotatable shaft 38, said shaft being rotatably mounted bya supporting element (not shown) that is fixed relative to whateversupports the fixed ends of bellows 21-24. To permit adjustment of nozzle35 by rotation of shaft 38, a knob 39 is provided on shaft 38 forturning the plate 37 and, hence nozzle 35. Also, piping is coiled andflexible, as shown, in order to permit the nozzle 35 to be deflectedabout the axis of shaft 38.

The parts of the nozzle mechanism are so positioned that when crossbaris in the aforesaid predetermined plane, the axis of thecircular-section rod 36 is coextensive with the axis of rotation ofnozzle 35 about shaft 38. This orientation of parts defines a neutralposition, in which, if knob 39 is turned, the nozzle opening moves aboutthe circular contour of rod 36 without the flow obstructing efifect ofrod 36 changing, due to the fact that the spacing between nozzle openingand the rod-surface stays constant.

As thus far described, controller 20 is old in the art, insofar as thisapplication is concerned, although in practice the adjustable baflle andnozzle mechanism are otherwise realized (to give however, the sameresults envisaged here). Controllers and relay devices of this type aredescribed and claimed in the prior copending application of H. R.Jaquith, Ser. No. 626,537, filed December 5, 1956, and assigned to theTaylor Instrument Companies, now US. Letters Patent No. 3,047,002,issued July 31, 1962.

Taking the parts in the position shown as defining the aforesaid neutralposition with the pressures in each of bellows 21 to 24 at 9 p.s.i.gauge, increase in the pressure in bellows 21 would throttle nozzle 35.Supposing the booster relay St? to be direct acting, the pressure inline 70 would be increased by the relay 30 and, supposing valve V to beof biased-closed and air-to-open type, it would increase its openingfrom whatever it was at the 9 psi. gauge position, (remembering thatoverride controls 0 are supposed to be relays that repeat the signalsapplied to them, in this case, air pressure).

Therefore, if transmitter S is designed so that it increases thepressure in bellows 21 in response to decrease in speed, (and vice versafor increase in speed) the result is that more fuel is fed to turbine Tand it attempts to restore its generator to a speed corresponding to 9p.s.i. gauge in bellows 21.

Considerations of load-sharing aside, for control purposes in general,bellows 23 and 24 and restrictions 27 and 23 are normally arranged tovary the control pressure applied to valve V in such a way as to restorethe original speed as stably and quickly as possible. That is,restrictors 27 and 28 respectively provide for reset and rate effects,as is well known in the art. In brief, a deflection of crossbar 25' outof the aforesaid reference plane, due, say, to pressure increase inbellows 21, increases the pressure in the motor of valve V and inbellows 23 and 24, the end result being increase in turbine speed.Hence, the pressures in bellows 21, 23 and 24 change in a mannerdetermined by, respectively, the time constant of the connectionsbetween relay 30 and bellows 23, the time constant of the connectionsbetween relay 30 and bellows 24, and the time constant involved inmaking a change in the output pressure of relay 30 result in a change inoutput of transmitter S into bellows 21.

By adjusting the values of restrictions 27 and 28, and the fluidcapacity involved in bellows 23 and 24 and their connections to relay 30to suit the process characteristics, namely: the manner in which theturbogenerator responds to electrical load changes, it is possible tomake turbine speed follow closely the Normal Load line (whichcorresponds to the pressure in bellows 22) and to cause the curvature bywhich turbine speed restabilizes upon change in load (i.e., thetransient characteristic of the system exemplified by the dashed linecurves in FIGURE 1) to conform to a known criterion of control qualitynot relevant here. This, however, is a typical approximation to controlwithout droop, i.e., isochronous governing of turbine speed, oroffset-less control. Hence, it is necessary to introduce the desireddroop somehow.

The prior art application of pneumatic control principles involvesproviding a pressure regulator, which can be set to produce a set pointpressure corresponding to the desired value of whatever process variableis being controlled. In this case, the process variable is turbinespeed, hence, the set point pressure in bellows 22 would be maintainedat a value corresponding to Normal Speed.

According to my invention, rather than connecting a set pointtransmitter or regulator directly to bellows 22, I connect a set pointtransmitter to a biasing or droop relay and the biasing relay to setpoint bellows 22. Such biasing relay must be of the type which can beadjustable to modify the value of the set point pressure by an amountcorresponding to the necessary droop and which retransmits the set pointpressure, thus modified, to bellows 22.

In FIGURE 3, reference numeral 12 denotes a set point transmitterconnected by pipe 17 to the relay 40. Transmitter 12 is supplied withair as indicated at A.S., and includes a knob 13 and structure (notshown) which responds to turning of knob 13 to produce from the supplyair, a set point pressure in pipe 17 that has a magnitude correspondingto the setting of knob 13. Suitable means in the nature of a set pointpressure gauge or knob position indicating means, and having a scale andpointer indicating means 14, is provided to permit setting the pressurein pipe 17 at the desired value. Conveniently the indications of means14 may be in terms. of turbine speed and, in this case, the speed scalereadings would be inversely proportional to the pressure in pipe 17.(Normally, a pressure gauge 15 having scale and pointer means 16 wouldalso be connected to pipe 60 to indicate actual turbine speed, asmeasured by transmitter S.) Under the circumstances outlined here, themeans 16 would be scaled off in turbine speed, the readings of whichwould be inversely proportional to the pressure in line 60.

The basic structure of [relay 40, which is the aforementioned biasing ordroop relay, is identical to that of controller 20. Specifically,bellows 41-44, crossbar 45 and flexible, inextensible element 45A areprovided and arranged exactly as their counterparts in controller 20,except for pressure connections. Likewise, the identical adjustablebafiie and nozzle mechanism is provided (omitting in FIGURE 3 all butbafile rod 46 and nozzle 47, for the sake of clarity). Also, a boosterrelay 50 supplied with air via a pipe 51 (which air is also applied viaorifice 52 and a pipe 53 to nozzle 47) is connected by a pipe 54 to pipe53 to allow relay 50 to sense nozzle back pressure and to produce acorresponding output pressure in a pipe 59 connected into a pipe 58connecting set point bellows 22 of controller 20 with a bellows 42 ofbias relay 40. Set point transmitter 12 having its out put pressureconnected via pipe 17 to bellows 41 direct- 1y opopsite bellows 42, itis evident that the pressure in bellows 41 will be reflected by apressure in bellows 42 that acts on crossbar 45 opposite to the pressurein bellows 41.

The bellows 44 is connected to atmosphere via a vent 19, and bellows 43is connected by pipe 18 to pipe 90,

7 in which reigns the control pressure which operates valve V.

Thus, as will be obvious from the foregoing, the pressures in bellows 41and 43 will determine, for a given angular position of nozzle 47relative to rod 46, the pressure in bellows 42 and 22, and, hence, thespeed set point or Normal Speed of the turbine T.

When the turbine T is 100% loaded by its generator, i.e., when thegenerator is taking its full share of the Normal Load, and the load isunchanging, fuel supply control valve V must be opened to apredetermined extent which in turn determines the control pressure inline 96 and bellows 43. At the same time, the setting of set pointtransmitter 12 will be such as toprovide a pressure in bellows 41corresponding to Normal Speed, and equal to the output pressure oftransmitter S which will be measuring actual speed.

Accordingly, the pressure in each bellows 22 and 42 must also be equalto the pressure in each of bellows 41 and 21, hence, under thesecircumstances, the crossbar and bellows structure of both controller 20and bias relay 40 are adjusted so that their crossbars 25 and 45 are inthe neutral position described, supra, in the case of controller 20,namely, such that the nozzles 35 and 47 can be deflected about theirrespective rods without changing the output of the respective boosterrelays.

Accordingly, with the turbo-generator running at Normal Speed under 100%of load, bias relay 46 simply repeats the set point pressure in bellows41 into set point bellows 22 of controller 20. However, if theelectrical load on the generator then changes, obviously booster relay30 will establish a new output pressure in pipe 29, which will berepeated via override controls in pipe 9%) and therefore into bellows 43as well as the motor of valve V. As a result, crossbar 45 will deflectand alter the position of rod 46 relative to nozzle 4'7. Nozzle 47,therefore, is initially set at such an angle relative to the long axisof rod 46, that deflection of crossbar 45 disturbs the spacing betweenrod 46 and the opening of nozzle 47. As a result, when the pressure inbellows 43 changes from the value corresponding to Normal Speed of theturbine, the pressure in bellows 42 will change in an amount and sensedepending on the angular setting of nozzle 47 and the sense and amountof such pressure change. This phenomenon supplies the droop.

In the angular position shown for nozzle 47, if the generator loadincreases from 100% of Normal Load, turbine speed will drop andcontroller 2% will increase its output pressure, thereby increasing thepressure in bellows 43, and rod 46 is therefore tilted toward theopening of nozzle 47. The resulting increase in nozzle back pressure issensed by booster relay $0, which in turn increases the pressure inbellows 22 and 42. Accordingly, rod is forced to move away from thenozzle opening, the eventual result being that the pressure in bellows42 increases just sufliciently to counterbalance the increase inpressure in bellows 43.

Since the pressure in bellows 22 also increases, rod 36 moves away fromthe opening of nozzle 35 and therefore opposes the original change inthe pressure in bellows 21 that began the events just described. Thatis, the back pressure of nozzle 35 decreases, booster relay 3%) sensesthe decrease, and decreases its output pressure into line 2s, and as aresult, valve V decreases the rate at which fuel is being supplied tothe turbine, thereby drooping the turbo-generator. The effect is someWhat as if knob 13 had been turned to establish a lower Normal Speedvalue, that is to increase the pressure in line 17, although of course,transmitter 12 still continues to establish in bellows 41 a pressurecorresponding to the Normal Speed for 100% of Normal Load.

The amount of pressure droop or bias required is normally a smallfraction of the set point pressure in bellows 41. Hence, the angularposition of nozzle 47 would be such that tilting of crossbar 45 bybellows 43 (which would be about an axis defined, in effect, by thecrossbar connections to bellows 41 and 42), varies net rod-nozzlespacing less than the same amount of tilt, applied by bellows 42 aboutthe axis determined by the crossbar connections to bellows 43 and 44,would vary net rod-nozzle spacing.

It will be noted, of course, that if load decreases from 100% of normal,the converse effect occurs, i.e., the pressure in bellows 22 decreasesbelow the value corresponding to Normal Speed for 100% of Normal Load,which prevents the generator from dropping load upon decrease of thetotal load from 100% of Normal Load. Obviously, too, if load returns to100% of Normal Load, the bias relay 40 restores the pressure in bellows22 to a value equal to that established in pipe 17 by set pointtransmitter 12.

Although I have gone into great detail as to the relays 20 and 40, etc.,I have done so mainly by way of example, and in order to indicateclearly the advantages of pneumatic instrumentation in the specificcontrol situa tion involved here.

Note, for example, that relays 20 and 40 are identical except forpiping. Likewise, relays 31B and 50 may be identical with each other.Indeed, override control 0 and transmitter S may have the same basicstructure as relays 20 and 40, although in practice it is not customaryto go so far for the sake of uniformity. Again, with reference to theoverall system, i.e., the generators 1 to n and the apparatus directlyappurtenant thereto: speed control system, override controls 0, turbine,etc., such system may be part of an even larger system wherein othercontrol functions are exercised by means of pneumatic controllers andrelays such as relays 20 and 40. It is easy to see that as a matter ofmaintenance, service, etc., it is within the province of my invention touse structurally quite diflerent (but functionally similar) types ofpneumatic relays than those illustrated in FIGURE 3. Again, it is notabsolutely necessary that the full complement of proportioning, resetand rate actions be applied. Normally, however, at least proportioningplus reset action, or equivalent would be provided, since although theinherent droop of a proportional-only control ler would tend toalleviate load sharing difflculties, it is more convenient to: introducedroop for the purpose in the manner disclosed herein, namely, by addingit to a system that would otherwise tend to control isochronously.

The purpose of controlling the bias action of droop relay 46 in responseto what amounts to output pressure of relay 3% is to make such biasaction reflect load changes in the electrical load. That is, variationsin the electrical load are reflected by variations in turbine speed,hence, measurement of turbine speed serves as a measurement of loadchange. It would also be possible to measure load changes otherwise thanindicated, so long as the measurement eventually results in a pressurewhich can be supplied to bellows 43 to do what the control pressure inpipe does in FIGURE 3.

The override controls 0 form no part of the invention and may or may notbe used, as desired. It is to be noted that whereas with a typicalhydraulic system, overrides are designed (to order, more or less), intothe control package, on the other hand, with pneumatic controlcomponents, no such package is involved, and the control system for aparticular turbo-generator unit is more or less built up ad lib bypiping together the sundry relays as shown. In practice, it is usual tointegrate booster relay, baifle nozzle mechanism and the basic relayunit together, likewise, some sort of easing structure housing gaugesand indicating or recording mechanisms such as speed indicator 15 andset point transmitter 12 may be provided, to which casing may beattached to the controller Zil.

Such integration is done on a plugin basis or quickdetach and attachbasis, however, not feasible with liquid type systems, hence, structuralunity of a pneumatic control system, lends itself to service and repairpractices that would be infeasible in the case of a hydraulic speedcontrol unit, unless an expert mechanic and special tools wereavailable. Thus, a faulty bias relay 40 or controller 20 need merely bereplaced bodily by breaking a few air connections and installing a newunit. In a hydraulic system, equivalent practices are far more tediousand troublesome, e.g., replacement of droop linkage is a task for anexpert and replacement of the controller corresponds to replacingelements corresponding to everything shown in FIGURE 3 but the valve V,that is, in short, the entire speed control unit.

I believe the foregoing demonstrates a considerable advance in the artand one that is worthy of patent protection. While the foregoing is aclear and fullydetailed description of the best mode of practicing myinvention, it is evident that modifications may be made therein withoutdeparting from the spirit orf the invention. Therefore, I have framedthe claims appended hereto accordingly and intend that they, rather thanthe details I have disclosed, supra, set the patentable bounds of myinvention.

I claim:

1. In combination, a generator and: a motor for driving said generatorat a given rate when said generator is generating into a given load;said generator being connected to a varying load; whereby said motortends to drive said generator at a varying rate inversely related tovariation in generator loading; relay means constructed and arranged forcomparison of. first and second signals and for producing a controlsignal corresponding to the difference between said first and secondsignals; measuring means responsive to the said varying rate forproviding said first signal in accordance with said varying rate; setpoint means settable to provide said second signal in accordance withsaid given rate; first signal transmitting means being connected betweensaid relay means and said measuring means for transmitting said firstsignal to said relay means; second signal transmitting means beingconnected between said relay means and said set point means fortransmitting said second signal to said relay means; all the aforesaidmeans being so arranged as to produce said control signal as aforesaid;motor control means responsive to said control signal for causing saidmotor to change said varying rate of drive in a sense tending to reducesaid difference between said first and second control signals; wherebysaid varying rate of drive tends to maintain a value corresponding tothe setting of said set point means; settable signal biasing meansconstructed and arranged to receive a signal and to retransmit suchsignal with a bias corresponding to the setting of said settable signalbiasing means; said settable signal biasing means being arranged in saidsecond signal transmitting connection for receiving and retransmittingsaid second signal, as aforesaid, to said relay means; said settablesignal biasing means being responsive to said varying load to adjustsaid bias in accordance with the difference between said given load andsaid varying load; said settable signal biasing means being soconstructed and arranged that for varying load difference from saidgiven load, said second signal is so biased as to be retransmitted withsuch value that the difference between said first signal and said secondsignal retransmitted is less than the dilierence between said firstsignal and said second signal received by said settable signal biasingmeans.

2. In combination, means responsive to actual frequency of generation ina motor-generator set for producing a variable pressure varying withactual frequency; means settable to produce a fixed pressurecorresponding to a desired frequency of generation in saidmotorgenerator set; a bias responsive to said fixed pressure to producea modified pressure equal to the sum of said fixed pressure and a biaspressure; relay means responsive to said variable pressure and saidmodified pressure to produce a control signal varying in accordance withthe relation between said modified pressure and said variable pressure;control means responsive to said control signal such as to tend to causesaid motor-generator set to generate at said desired frequency; thearrangement being that said control signal varies in a sense such as totend to cause said actual frequency to change in a sense opposingchanges of actual frequency causing said control signal to vary; saidbias relay also being responsive to said control signal so as to addsaid bias pressure to said fixed pressure in such sense as to reduce thetendency of said control signal to cause said actual frequency to assumethe value of said fixed frequency.

3. In combination, a process variable transmitter for producing aprocess variable signal in accordance with a process variable; a setpoint transmitter settable to produce a set point signal in accordancewith its setting; a controller; a first connection means connecting saidprocess variable transmitter to said controller; a second connectionmeans connecting said set point transmitter to said controller; saidfirst and second connection means providing said controller with,respectively, said process variable signal from said process variabletransmitter and said set point signal from said set point transmitter;said controller being constructed and arranged to produce a controlsignal variable in sense and magnitude according with the differencebetween a signal provided it by said first connection means and a signalprovided it by said second connection means; bias means in said secondconnection means, said bias means being settable to add a bias to saidset point signal; whereby said control signal varies in sense andmagnitude in accordance with the difference between said processvariable signal and said set point signal biased by said bias means.

4. A droop-type relay system including, a controller constructed andarranged to have an input for receiving a process variable input signal,an input for receiving a set point input signal, and an output at whichto produce an output signal; said controller being responsive to thedifference between said input signals such as to change its said outputsignal in an amount and sense determined by the amount and sense of thedifference between the said input signals; and a droop relay having aninput connected to said output of said controller for receiving saidoutput signal; an input for receiving said set point signal, and anoutput at which to produce a modified set point input signal, said drooprelay being constructed and arranged to be responsive to deviation ofsaid output signal from a given value to produce at its said output asaid modified set point input signal as a function both of the firstsaid set point input signal and of said deviation of said output signal;said output of said droop relay bemg connected to said controllerssecond said input for providing said second said input with saidmodified set point input signal. 5. The invention of claim 4, whereinsaid controller includes a pneumatic means responsive to the signalsapplied to said controllers said inputs to produce said output signal inthe form of an equivalent pneumatic output signal; said droop relayincludes pneumatic means responsive to a pneumatic signal applied to thefirst said input of said droop relay to produce at its said output thesaid modified set point input signal, and there being a pneumaticconnection connecting said output of said controller to the said firstsaid input of said droop relay for applying said pneumatic output signalto said first said input of said droop relay.

6. A power-generating system including: a power generator; a variablespeed motor for driving said generator; control means for exerting aspeed adjusting efiect on said motor; measuring means for measuring thespeed of said motor and producing a first effect corresponding to actualmotor speed; set point means for setting the speed of said motor and forproducing a second effect in accordance with its setting; said measuringmeans and said set point means being automatically operative to causesaid control means to exert said speed-adjusting effect on said motor insuch fashion as to oppose change in motor speed, said speed-adjustingefiect corresponding to the difference between said first and secondeffect; said measuring means being constructed and arranged to producesaid first effect in the form of a pneumatic signal corresponding to thesaid speed of said motor, and said control means being constructed andarranged to respond to said pneumatic signal and to said second efiectin order to produce said speed-adjusting effect; droop means responsiveto said speed-adjusting effect for modifying said second effect so as tooppose the tendency of said speed-adjusting effect to oppose saidchanges in motor 5 speed.

7. The invention of claim 6, said droop means being responsive to saidset point means to produce said second efiect, and said droop means alsobeing responsive to deviate from accordance with said setting of saidset point means; said droop means being constructed and arranged so thatsuch deviation of said second effect from accordance with said settingis in accordance with difierence between actual motor speed and themotor speed corresponding to said setting, but has a sense such as tomake the diiference between said first effect and said second effectless than the said difference between actual motor speed and the motorspeed corresponding to said 10 setting.

References Cited by the Examiner UNITED STATES PATENTS 2,273,407 2/42Lilja 29040.1 2,811,651 10/57 Lepley 29040 FOREIGN PATENTS 583,247 9/59Canada.

to said speed-adjusting effect to cause said second effect 20 ORIS L.RADER, Primary Examiner.

6. A POWER-GENERATING SYSTEM INCLUDING: A POWER GENERATOR; A VARIABLESPEED MOTOR FOR DRIVING SAID GENERATOR; CONTROL MEANS FOR EXERTING ASPEED ADJUSTING EFFECT ON SAID MOTOR; MEASURING MEANS FOR MEASURING THESPEED OF SAID MOTOR AND PRODUCING A FIRST EFFECT CORRESPONDING TO ACTUALMOTOR SPEED; SET POINT MEANS FOR SETTING THE SPEED OF SAID MOTOR AND FORPRODUCING A SECOND EFFECT IN ACCORDANCE WITH ITS SETTING; SAID MEASURINGMEANS AND SAID SET POINT MEANS BEING AUTOMATICALLY OPERATIVE TO CAUSESAID CONTROL MEANS TO EXERT SAID SPEED-ADJUSTING EFFECT ON SAID MOTOR INSUCH FASHION AS TO OPPOSED CHANGE IN MOTOR SPEED, SAID SPEED-ADJUSTINGEFFECT CORRESPONDING TO THE DIFFERENCE BETWEEN SAID FIRST AND SECONDEFFECT; SAID MEASURING MEANS BEING CONSTRUCTED AND ARRANGED TO PRODUCESAID FIRST EFFECT IN THE FORM OF A PNEUMATIC SIGNAL CORRESPONDING TO THESAID SPEED OF SAID MOTOR, AND SAID CONTROL MEANS BEING CONSTRUCTED ANDARRANGED TO RESPOND TO SAID PNEUMATIC SIGNAL AND TO SAID SECOND EFFECTIN ORDER TO PRODUCE SAID SPEED-ADJUSTING EFFECT; DROOP MEANS RESPONSIVETO SAID SPEED-ADJUSTING EFFECT FOR MODIFYING SAID SECOND EFFECT SO AS TOOPPOSE THE TENDENCY OF SAID SPEED-ADJUSTING EFFECT TO OPPOSE SAIDCHANGES IN MOTOR SPEED.